Datasheet

7
LT1108
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
315
19
16 6 07.()µ
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
I
V
emA
PEAK
s
H
=






=
×
2
10
1 605 08
10 36
100
.
–()
–.
Ω
Ωµ
µ
Substituting I
PEAK
into Equation 04 results in
EHAJ
L
=
()( )
=
1
2
100 6 605 18 3 09
2
µµ.. ()
Since 18.3µJ > 16.6µJ, the 100µH inductor will work. This
trial-and-error approach can be used to select the optimum
inductor. Keep in mind the switch current maximum rating
of 1.5A. If the calculated peak current exceeds this, an
external power transistor can be used.
A resistor can be added in series with the I
LIM
pin to invoke
switch current limit. The resistor should be picked so the
calculated I
PEAK
at minimum V
IN
is equal to the Maximum
Switch Current (from Typical Performance Characteristic
curves). Then, as V
IN
increases, switch current is held
constant, resulting in increasing efficiency.
Step-Down Converter
The step-down case (Figure 2) differs from the step-up in
that the inductor current flows through the load during both
the charge and discharge periods of the inductor. Current
through the switch should be limited to ~650mA in this
mode. Higher current can be obtained by using an external
switch (see Figure 3). The I
LIM
pin is the key to successful
operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
I
I
DC
VV
VV V
PEAK
OUT OUT D
IN SW D
=
+
+






2
10
–
()
where DC = duty cycle (0.60)
V
SW
= switch drop in step-down mode
V
D
= diode drop (0.5V for a 1N5818)
I
OUT
= output current
V
OUT
= output voltage
V
IN
= minimum input voltage
V
SW
is actually a function of switch current which is in turn
a function of V
IN
, L, time, and V
OUT
. To simplify, 1.5V can
be used for V
SW
as a very conservative value.
Once I
PEAK
is known, inductor value can be derived from
L
VVV
I
t
IN MIN SW OUT
PEAK
ON
=
−−
× ()11
where t
ON
= switch-ON time (36µs).
Next, the current limit resistor R
LIM
is selected to give I
PEAK
from the R
LIM
Step-Down Mode curve. The addition of this
resistor keeps maximum switch current constant as the
input voltage is increased.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
I
mA
mA
PEAK
=
()
+
+






=
2 300
060
505
12 15 05
500 12
.
.
–. .
()
Next, inductor value is calculated using Equation (11)
L
mA
sH==
12 1 5 5
500
36 396 13
–.–
()µµ
Use the next lowest standard value (330µH).
Then pick R
LIM
from the curve. For I
PEAK
= 500mA,
R
LIM
= 220Ω.
Positive-to-Negative Converter
Figure 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
P
L
= (V
OUT
+ V
D
)(I
OUT
) (14)